Disclamer: this matters to cash shop users only.
When enhancing accessories, if you do not already have enough resotration scrolls for the inevitable failure - sometimes you cannot purchase restoration scrolls during enhance failure - only the limited/day black pearl scrolls can be bought. I was lucky that i only needed 2 scrolls otherwise i would have lost a +7 purple accessory due to the purchase icon for white pearl restoration scrolls being absent.

Side note - Restarting the game fixes it, but could be a disaster if no scrolls can be purchased during an enhance failure